9. Description/prediction of element accumulation in the unsaturated zones of arid and semi-arid regions

The chemistry and hydrology of soil and the greater unsaturated zone beneath semi-arid and arid lands in the western United States are topics whose importance will increase with anticipated land-use and climate change. A product of natural processes in these dry regions is the widespread existence of sulfate, nitrate, chloride, and carbonate salts in the subsurface. These salt accumulations, some of which have been forming for thousands of years, result from a combination of climatic, geologic, hydrologic, and biologic processes (Rengasamy, 2006). Natural sources of ions contained in these salts include the enclosing sediments, ground water and atmospheric deposition. Changing land use, including agriculture and urban development, waste disposal by land spreading, as well as climate change can modify natural conditions affecting the stability and position of these salt enrichments. Dissolution of the salts may adversely impact water quality in arid ecosystems. In addition to increasing salinity, hazardous constituents such as arsenic, selenium and uranium can be trapped in the salt so that even partial dissolution can degrade important aquifers and surface water bodies.

Examples of salt impacts are reported for many semi-arid landscapes. In the Powder River Basin of Wyoming, water produced during extraction of coal-bed methane is placed in surface storage impoundments. For at least one such impoundment, infiltrating water has dissolved salts and created a perched layer of saline water in the subsurface. In addition to movement through groundwater, air borne transport of salts formed by evaporation of shallow ground water can produce dust storms bearing metal-rich salts, such as those from Owens (dry) Lake, California. In Australia’s Murray Basin, replacement of native vegetation with agricultural crops has led to reduced evapotranspiration rates and increased rates of ground-water recharge. The increased water flow through the unsaturated zone has flushed out salts, raised groundwater levels, and increased the discharge of a saline groundwater into the Murray River, which is a major source of water for towns and cities along its course.

Steep hydraulic and chemical gradients characterize the unsaturated zones of arid and semi-arid regions. Existing research has produced descriptions and mathematical simulations of water movement through this zone; but redistribution of major and trace elements in response to wetting fronts, capillary rise and vapor movement remains poorly understood. The current lack of process-level understanding of coupled chemical and hydrologic processes limits our ability to accurately describe and forecast how and where salts accumulate and how they will respond to changes in rates of water movement through the unsaturated zone. They could increase in abundance or change location, as well as be transferred to surface or ground waters and eventually affect distant ecosystems.

The goal of research under this opportunity is to develop tools for predicting the rates of salt accumulation and removal. The postdoctoral research will be directed toward establishing a framework of the processes that control the formation, accumulation, and movement of major and trace elements in the unsaturated zone. This new knowledge will then be coupled with our existing understanding of water movement. Laboratory and field studies will be augmented by computer-based simulations. Parameters relevant to this research include the physical and chemical characteristics of the substrate, rates of water movement, nucleation and growth of salt, chemical characteristics of accumulated phases, mass transfer of constituents by advection and diffusion in the unsaturated zone, climate, vegetation, and land use. An eventual application of the research would be to develop the capability to predict salt behavior over large and varied geographic areas.

REFERENCE

Rengasamy, Pichu, 2006, World salinization with emphasis on Australia: Journal of Experimental Botany v. 57, p. 1017-1023.

Proposed Duty Station: Denver, CO

Areas of Ph.D.: Geochemistry, hydrology, soil science, geomorphology, environmental science, or geology

Qualifications: Research Chemist, Research Soil Scientist, Research Hydrologist, Research Geologist, Research Physical Scientist

(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)

Research Advisor(s): George Breit, (303) 236-4951, gbreit@usgs.gov; Richard Healy, (303) 236-5392, rwhealy@usgs.gov; Marith Reheis, (303) 236-1270, mreheis@usgs.gov; Geneva Chong, (307) 733-9212, ext. 226, geneva_chong@usgs.gov; Tim Munday (CSIRO, Australia), (08) 6436-8634 , Tim.Munday@csiro.au

Human Resources Office contact: Kathleen Scheich, (303) 236-9581, kscheich@usgs.gov

Summary of Opportunities